1. Technical Field
The present invention relates to a temperature-measuring device that measures temperature of a sheet and an image-forming apparatus that uses such a temperature-measuring device.
2. Background Art
An image-forming apparatus adopting an electrophotographic system such as a printer, a copier, a facsimile or the like has been widely utilized in the past. In the image-forming apparatus, there may be variations in environmental conditions such as temperature, humidity or the like and any deterioration of photoreceptor, developing material or the like with time, namely, a variation in concentration of image and/or printing position based on the durability deterioration. Therefore, the image-forming apparatus may fail to form an image stably. Accordingly, the image-forming apparatus contains a color sensor provided on a downstream side of a fixing device. The color sensor has detected patches for image adjustment formed on a sheet. This detection result has been fed back to an image-forming condition such as exposure, charge, development bias or the like so that a control for stabilizing the image (image-stabilizing control) can be performed.
However, in a past image-stabilizing control, the color sensor detects the patches formed on a sheet passed through the fixing device. In this case, thermochromism phenomenon may occur by changing temperature of the sheet or the patches by heating of the fixing device, thereby shifting color measurement values of the patches to be measured by the color sensor from a correct color measurement value thereof. Here, the thermochromism phenomenon is referred to as a phenomenon in which a color of a toner image becomes changed by changing a chemical bond of the elements in toner materials by heating.
In view of the above, a control has been performed in which an infrared radiation temperature measurement sensor such as a thermopile sensor measures temperature of a sheet and color measurement values of the patches for image adjustment are corrected on the basis of the sheet temperature acquired by this measurement. According to this control, it has been possible to perform any correct color measurement independent of any temperature of the sheet. Therefore, in the above control, the correct measurement of the temperature of the sheet P has been important. For example, Japanese Patent Application Publication No. 2004-157009 discloses a radiation temperature measurement method that is capable of measuring temperature, without being directly influenced by the variations and changes in the sensitivity of a thermal radiation sensor. Japanese Patent Application Publication No. H07-324981 discloses a radiation thermometer that is capable of improving efficiency of setting operation of radiation when measuring temperature.
A past infrared radiation temperature measurement sensor disclosed in Japanese Patent Application Publication No. 2004-157009 or Japanese Patent Application Publication No. H07-324981 has the following characteristics.
FIG. 1 shows characteristics of a thermopile sensor as the infrared radiation temperature measurement sensor. A horizontal axis thereof indicates a sheet temperature measured by the thermocouple of contact type and a vertical axis thereof indicates a difference between the sheet temperature measured by the thermocouple of contact type and the sheet temperature measured by the thermopile sensor of contactless type (hereinafter, referred to as “temperature difference between the thermocouple and the thermopile sensor”). It is to be noted that sheets A, B and C have different paper weights and different surface characteristics. As shown in FIG. 1, when the sheet temperature measured by the thermocouple is 55 degrees C., the temperature differences between the thermocouple and the thermopile sensor are about +1 degree C. in the sheet A, about −1 degree C. in the sheet B and about −3 degrees C. in the sheet C. For example, it is known that when the sheet temperature is shifted by 4.9 degrees C., the color measurement value is shifted to ΔE76=1.00.
Thus, when measuring the sheet temperature using the thermopile sensor of contactless type, this sheet temperature may be very different from the temperature measured by the thermocouple because radiations differ for every species of sheet. Accordingly, there may be a case that is insufficient to meet a condition of temperature difference (for example, ±3 degrees C.) between the thermocouple and the thermopile sensor, which is required as a precision of the color measurement value. It is conceivable to correct the radiation according to the species of sheets to be measured as the past. There, however, are enormous species of sheets in a modern market and it is difficult in a realistic manner to correct the radiation for every species of sheet. Further, it is difficult to measure the sheet temperature in the precision required for any correction of the thermochromism phenomenon.